Space achievements rarely happen on time. The launch date for SpaceX's Demo-1 mission of its Crew Dragon got pushed back repeatedly. Boeing's Starliner is seeing a similar pattern.

NASA announced on Wednesday that Boeing will target August for the first uncrewed Orbital Flight Test of the CST-100 Starliner. Boeing had previously looked at launching in March. That date was later pushed into April. A crewed mission is unlikely to happen until late 2019.

NASA voiced a vote of confidence in the Starliner systems, saying the agency and Boeing have agreed to extend the duration of the eventual first crewed test flight to the ISS.

"The extended duration test flight offers NASA the opportunity to complete additional microgravity research, maintenance, and other activities while the company's Starliner is docked to station," says NASA, though it hasn't set the exact duration yet.

The August Starliner launch date is still tentative. A number of factors influence NASA launch schedules, and the agency says the new timing is partly due to limited launch opportunities in April and May. The Starliner spacecraft is close to completion and is designed to be reusable up to 10 times.

NASA hasn't yet set a solid date for a crewed SpaceX Demo-2 mission to the station. The space agency says it will take a fresh look at target launch dates over the next couple of weeks.

The test flights are serious milestones for the Commercial Crew Program. NASA has been relying on rides on Roscosmos Soyuz spacecraft to ferry astronauts to the ISS. SpaceX and Boeing expect to bring that capability back to the US for the first time since the space shuttle program ended in 2011.